Traditional methods for the manufacture of metal parts include, for example, machining from forging, bar stock or tube. However, these traditional methods of manufacture have poor material utilization and relatively high cost versus production by Powder Metallurgy (PM) processes. Other advantages with PM processes include the ability to form complex shapes in a single forming operation, minimal finish machining, high volume capacity and energy efficiency.
Notwithstanding the advantages referred to above, the utilization of PM sintered parts in automobiles is still relatively modest when compared to low alloy wrought steel. One area of future growth in the utilization of PM parts in the automotive industry resides in the successful entry of PM parts into more demanding applications, such as power transmission applications, for example, transmission gears. One problem with gear wheels formed by the PM process in the past has been that powder metal gears have reduced bending fatigue strength in the tooth and root region of the gear, and low wear resistance on the tooth flanks due to the residual porosity in the microstructure versus gears machined from bar stock or forgings. One method of successfully producing PM transmission gears resides in rolling the gear profile to densify the surface as shown in GB 2250227B. However, this process teaches a core density which is lower than that in the densified regions, which is typically at around 90% of full theoretical density of wrought steel. This results in a tooth with comparatively lower bending fatigue endurance than its machined wrought steel counterpart.
Although sintering temperature can have a significant influence on dynamic properties of a sintered PM part at a given density, the ultimate dynamic property levels attainable for any sintering regime is also controlled by the combination of alloying system used and sintered density attained. Although it is possible to obtain high tensile strength with typical PM processes (with or without heat treatment) at single pressed density levels of up to 7.2 g/cm3, dynamic properties such as fracture toughness and fatigue endurance under cyclic loading will invariably be less than those of steel of comparable strength. Therefore, processes for the production of PM transmission gears have not gained wide support. This is primarily due to the negative effects of residual porosity. Accordingly, processes to improve properties of PM parts subjected to high loading must consider densification and microstructure of the highly loaded regions for good cyclic bending endurance and surface endurance respectively.
Methods for improving the properties of PM parts are known from the U.S. Pat. Nos. 5,729,822, 5,540,883 and 5,997,805.
U.S. Pat. No. 5,729,822 discloses a method of manufacturing PM components, useful for gears, comprising the steps of: a) sintering a powder metal blank to produce a core density of between 7,4 to 7,6 g/cm3; b) rolling the surface of the gear blank to densify the surface; c) heating the rolled sintered gear and carburizing in a vacuum furnace.
U.S. Pat. No. 5,540,883 discloses a method of producing PM components, useful for bearings, comprising the steps of: a) blending carbon, ferro alloy powder and a lubricant with compressible iron powder to form a blended mixture; b) pressing the blended mixture to form the article; c) sintering the article; d) roll forming at least part of a surface of the article with rollers and e) heat treating the layer.
U.S. Pat. No. 5,997,805 discloses a method of producing high density, high carbon, sintered PM steels. The method includes: blending powders of desired composition; compacting and sintering the powder; cooling the sintered article by isothermal hold or slow cooling; followed by forming the article to a density between 7,4 to 7,7 g/cm3. By cooling the sintered article followed by isothermal hold a lower hardness of the high carbon material is obtained for the following forming operation.
The present invention provides a new method for producing PM components with a core distinguished by medium to high density, high yield strength and a surface with high hardness and high density.